Abdalla Salem El-Badri, Secretary General of the OPEC from Libya, speaks at a news conference of the Organization of the Petroleum Exporting Countries, OPEC, on the occasion of their 50th anniversary at their headquarters in Vienna, Austria, on Tuesday, Sept. 14, 2010. (AP Photo/Ronald Zak) | AP

An independent Scotland would struggle to be economically self-reliant as most of its North Sea oil reserves are "depleted", the head of the Organisation of Petroleum Exporting Countries (OPEC) has warned.

Commenting personally on Scottish independence, OPEC secretary general Abdalla Salem el-Badri said Scotland would be best off remaining in the United Kingdom.

"Myself, I don't see how England will stay without Scotland and Scotland stay without England. This is how we all grew up, with the UK, not with Scotland and England," he told the Daily Telegraph.

Badri admitted that the Scottish referendum had "nothing to do with OPEC", which represents the world's 12 oil-producing countries and controls a third of the world supply. However, he said: “Most of their reservoir is depleted now.”

Scottish first minister Alex Salmond has used the tax revenues from the North Sea reserves as a central argument for the country's independence.

However, former Labour chancellor Alistair Darling poured scorn on Salmond basing his argument on North Sea oil revenues, writing on HuffPostUK: "Oil has always been central to the nationalist case for independence. It has been used to reassure us that leaving behind the strength and security of being a part of the UK was not such a big risk in an uncertain and unstable world."

"The inconvenient truth which the SNP have always struggled to deal with is that all the revenues from the North Sea currently go towards spending on public services, pensions and benefits in Scotland.

"As the impartial Institute of Fiscal Studies found again a few days ago, public spending is £1,200 higher per head in Scotland than across the UK. It is this higher spending that stands behind the success of the Scottish Parliament."